The drop-on-demand ink-jet head is known as an ink-jet head that can eject a required amounts of ink droplets in response to the input signal, only when they are needed to print on the medium. In particular, extensive research is being undertaken on the piezoelectric (piezo) drop-on-demand ink-jet head as it is capable of well-controlled discharge of a wide variety of inks. The piezo drop-on-demand ink-jet head generally includes an ink supply channel; a plurality of ink chambers with a nozzle, which are connected to the ink supply channel; and piezoelectric elements for applying a pressure to the ink in the ink chambers.
In such a piezo drop-on-demand ink-jet head, piezoelectric elements deform to apply a pressure to the ink in the ink chamber in response to application of a drive voltage, so as to allow the ink to be discharged from nozzles. Broadly, there are three types of piezo drop-on-demand ink-jet head according to the manner in which the piezoelectric element deforms: shear mode, push mode and bend mode. In particular, because of its ability to produce high power at low voltage, the bend-mode piezo ink-jet head that uses multilayer piezoelectric elements is expected to be further developed for manufacturing of electric devices in which a highly viscous ink is printed, examples of the electric devices include an organic EL display panels and a liquid crystal panels.
FIG. 1 is a partially cross-sectional enlarged view of ink-jet head 10 disclosed in the Patent Literature 1. As illustrated in FIG. 1, ink-jet head 10 has an ink supply channel (not shown), a plurality of ink chambers 2b having nozzle 3a, and piezo-mounting plate 40 having multilayer piezoelectric elements 5f and columns 5g provided alternately.
Each ink chamber 2b is partitioned by partition walls 2d. Multilayer piezoelectric elements 5f vibrate vibrating plate (diaphragm) 6a constituting wall surfaces of ink chambers 2b. In ink-jet head 10, a plurality of ink chambers 2b shares the single diaphragm 6a. Diaphragm 6a is fixed by being held between columns 5g and partition walls 2d. 
In the above ink-jet head 10, multilayer piezoelectric elements 5f vibrate diaphragm 6a constituting the wall surfaces of ink chambers 2b, whereby the ink in ink chamber 2b is pressurized to be ejected through nozzle 3a. 
Further, in ink-jet head 10, diaphragm 6a is fixed by being held between columns 5g and partition walls 2d. Diaphragm 6a is fixed thus, whereby the vibration in ink chamber 2b is prevented from transmitting to the adjacent ink chamber 2b via diaphragm 6a (the transmitting of the vibration is a so-called “crosstalk”).
The resolution of a printer with the above ink-jet head 10 is determined by a nozzle-to-nozzle pitch P1 (hereinafter also referred to simply as “a nozzle pitch”). Specifically, the longer the nozzle pitch P1, the lower the resolution, and the shorter the nozzle pitch P1, the higher the resolution.
Patent literatures other than the Patent Literature 1 also disclose the technique of fixing a diaphragm by holding the diaphragm between columns (dummy sections) and partition walls (for example, see Patent Literatures 2 and 3).
Further, there has been proposed an ink-jet head having hexagonal columnar piezoelectric elements (for example, see Patent Literature 4). In the ink-jet head described in the Patent Literature 4, a voltage is applied to both the hexagonal columnar piezoelectric elements and triangular prism shaped piezoelectric elements arranged between the hexagonal columnar piezoelectric elements. Furthermore, another technique of manufacturing hexagonal columnar piezoelectric elements has been proposed (for example, see Patent Literatures 5 and 6).
Furthermore, there has been known a hexagonal columnar multilayer piezoelectric element having a hollow space therein, the hollow space being for an ink chamber (for example, see Patent Literature 7).
Ink-jet heads sometimes encounter the problem of failing to accurately discharge ink droplets due to air inclusion or nozzle clogging. To overcome this drawback, there have been proposed techniques in which ink is allowed to circulate through the ink-jet head so as to prevent air inclusion and nozzle clogging, the circulating means ink-feeding into and ink discharging from ink chambers (see, e.g., Patent Literature 8).